Recently digital copying machines are provided with not only a normal copy mode in which a document image read by a reading section of such a copying machine is printed on a copying sheet, but also a printer mode, a fax mode, etc., in which image data received from an external data processing device are printed. In the printer mode, image data are received from an external data processing device such as a computer (including a personal computer), a word processor, etc., and printed on a copying sheet as an image. In the fax mode, the image data are received from an external image processing apparatus through communication means such as a telephone line, and printed on a copying sheet.
In general, in the normal copy mode, sheets are sequentially discharged in a state in which a surface of the sheet on which an image has been printed faces upward (the face-up state). On the other hand, in the printer mode or the fax mode, sheets are discharged in a state in which the surface having the image formed thereon faces downward (the face-down state).
Namely, in the normal copy mode, documents are controlled to be sent to the reading section of the copying machine in sequence from the last page of the documents in consideration of the page order when sheets are discharged after the formation of the images. In this case, an automatic document feeder provided in the copying machine is used. Thus, printed materials (hard copies) of the document images are discharged in sequence in the same page order as the document.
On the other hand, when the digital copying machine functions as a printer or a facsimile machine, in general, image data are sent in sequence from the top page of the document from an external apparatus like a personal computer, a word processor, a facsimile machine, etc. Therefore, when discharging the sheets in sequence from the discharge section, if the received image data are printed on the copying sheets and then the sheets are stacked in the face-up state on the discharge trays, the printed materials are output in reverse page order.
In order to solve such a problem, in general, it has been proposed to arrange the printed materials in page order by turning over the sheets being discharged in the described state and stacking the sheets with their image-formed sides facing downward on the discharge section.
Such a discharging function is enabled by, for example, an apparatus disclosed in Japanese Publication for Unexamined Patent Application No. 310357/1993 (Tokukaihei 5-310357). The structure of the apparatus will be briefly explained below.
As described in the above-identified Japanese publication (see also FIGS. 1 and 3-4 thereof), when a toner image on a photoreceptor is transferred to a copying sheet sent from a feed tray, the copying sheet is discharged out of the apparatus through a fusing device. The apparatus is provided with a discharge processing unit for switching the discharge state of a sheet according to a mode selected, i.e., a copy mode or a printer mode. When the image forming apparatus is set in the copy mode, the sheet is discharged with its image-formed side facing upward onto a discharge tray from a discharge opening through a predetermined transport path in the discharge processing unit. On the other hand, in the printer mode, the transport path is switched, and the sheet is once guided to a switchback transport path through a predetermined transport path. Thereafter, the transport direction is switched so that the sheet is discharged with its image-formed side facing downward from the discharge opening onto another discharge tray provided below the above-mentioned discharge tray.
As switch means for switching the transport direction of a sheet in the transport path, the structure where members like the switching members are provided in the transport path to switch ON/OFF solenoids is adopted.
Japanese Publication for Unexamined Patent Publication No. 180869/1987 (Tokukaisho 62-180869) proposes to form a discharge section provided with a sheet discharging apparatus having a sorting function for discharging a sheet having an image printed thereon from a page printer to one of a plurality output bins after turning over the sheet. As described in this publication (see also FIG. 1 thereof), sheets sent to a sorter through a fusing device and a discharge roller of a page printer are temporarily guided to reverse rollers by transport rollers. Thereafter, the transport direction of the sheets is reversed in a predetermined state, and then the sheets are discharged to bins (BN.sub.1 to BN.sub.n-1), respectively. As a result, the sheets having the images printed thereon are discharged in proper page order.
In the structure described in the above-mentioned publication (Tokukaihei 5-310357), it is necessary to provide a digital image forming apparatus with a switchback transport path and the mechanism therefor as a sheet discharging apparatus for switching the transport direction of sheets having images formed thereon.
The provision of the switchback transport path and the mechanism cause an increase in the overall size of the image forming apparatus, thereby presenting a problem that a large space is required for the installation of the apparatus in an office. Moreover, it is necessary to provide a switchback transport path and a transport mechanism, which can reverse a sheet of the maximum printable size in the image forming apparatus. Consequently, the device tends to be gigantic, causing a disadvantage in terms of costs.
Moreover, during a period of time in which the transport direction of the preceding sheet is reversed in the switchback transport path and then transported to a predetermined bin (tray), the following sheet is kept waited. When the preceding sheet has been transported to the predetermined bin, the following sheet is transported to its switchback transport path. Namely, the next sheet cannot be fed during the reverse transport operation, and therefore the transport interval between sheets becomes longer. This prevents an increase in the output speed. In particular, the discharging operation cannot be performed according to the processing speed of the image forming apparatus. Thus, there is a need to adjust the processing speed of the image forming apparatus to meet with the processing speed of the switchback transport path.
Furthermore, according to the above-mentioned publication (Tokukaisho 62-180869), since a mechanism used exclusively for reversing the sheet is provided in the sorter as the sheet discharging apparatus as well as a plurality of bins (trays) for storing sheets, the increase in the size of the image forming apparatus, i.e., the printer, is prevented. However, in the structure where the reversing mechanism is provided in the sorter in addition to the sorting function, the reversing mechanism cannot be used as the sheet discharge section. Namely, since the reversing mechanism is used exclusively for turning over a sheet when discharging sheets, the reversing mechanism becomes a useless member for the discharge section unless the reversing mechanism is used.
Additionally, since the sheet is always turned over in a predetermined reversing section, it may take a long time to feed a sheet to a predetermined tray.
Moreover, when the preceding sheet is being reversed by the reversing mechanism, the next sheet is kept waited before the reversing mechanism, and the waiting sheet is fed to the reversing mechanism after the completion of the reversal of the preceding sheet. Consequently, like the device disclosed in the former publication (Tokukaihei 5-310357), the distance between the discharging position and the next sheet to be discharged becomes longer. It is therefore necessary to adjust the output speed of the printer to meet with the discharge processing speed. In particular, when the processing speed of the printer is faster than the discharge processing speed, a considerable economical disadvantage is caused, and the printer is provided with an excessively high processing speed.